The influence of dietary or pharmacological intervention on coronary atherosclerosis can best be assessed by performing serial coronary arteriography on patients at high risk for coronary disease progression who are randomised into controlled trials. Changes in coronary artery lesion diameter of less than 20% cannot be accurately assessed by visual interpretation, because of inter- and intra-observer variability. The introduction of quantitative, computer-assisted measurements of coronary lesions has reduced variability and greatly improved the precision of measurements. Both cholestyramine and colestipol-niacin have been shown to reduce the rate of progression of coronary atherosclerosis; however, in both studies coronary lesions were assessed visually and no quantitative measurements were reported. A marked reduction in serum cholesterol appears to be the intervention that is most likely to prevent the progression or induce regression of coronary atherosclerosis. Quantitative angiographic measurement techniques should be used in clinical trials designed to assess cholesterol-lowering interventions.